Findings from three major world observatories confirm that a very rare binary asteroid came within a cosmic whisker of Earth on June 21, 2017. The near-Earth asteroid 2017 YE5 (actually two asteroids 3,000 ft in diameter orbiting one another) passed within 3.7 million miles of our planet for the first time in 170 years before heading out into deep space.
Asteroids come in all shapes in sizes from dust motes to dwarf planets like Ceres, but another thing that sets them apart from one another is their orbital arrangements. According to NASA, of the family of near-Earth asteroids larger than 650 ft (200 m) in diameter, approximately 15 percent consist of binary asteroids like Didymos, where one smaller asteroid circles around a larger one.
Such a system is similar to the Earth and Moon, where our smaller satellite revolves around the planet. But this isn't the only way a binary asteroid can be put together. When we say that the Moon revolves around the Earth, it's really a less pedantic way of saying that Earth and Moon orbit around two shared orbital foci. That is, the two points inside an ellipse where the gravitational forces working on the Earth and Moon are centered.
Because the Moon is in a nearly circular orbit, these foci are very close together, and because the Earth is much more massive than the Moon, the foci are almost where the center of the Earth is situated. This means that the Moon orbits the Earth, while the Earth's contribution is a bit of a wobble.
The same situation goes for a system like Didymos, where a very large asteroid has a much smaller one orbiting it. But if both asteroids are of nearly equal size then the foci of their orbit sit in empty space, with neither asteroid acting as the center. 2017 YE5 is one such system – a type of object representing so tiny a percentage of the near-Earth family that only four have been discovered to date.
Another 15 percent of the near-Earth family are contact asteroids, which have come so close together that they touch. If two planets tried that, the result would be an explosive collision with one or both planets being ripped apart by tidal forces. But asteroids are too small for that, so they can survive the encounter if they're solid enough and not traveling too fast.
2017 YE5 was first sighted by the Morocco Oukaimeden Sky Survey on December 21, 2017, but at that point it was just a smudge on an image and nothing much could be discerned about its properties. However, on June 21, 2018, as the asteroid came within 16 times the distance of the Earth to the Moon, astronomers at NASA's Goldstone Solar System Radar (GSSR) in California took advantage of the opportunity and conducted a series of radar observations.
What they found was that 2017 YE5 showed signs of being a binary by presenting two lobes, which longer observations through June 22 proved to have a distinct gap between them.
Goldstone them alerted the Arecibo Observatory in Puerto Rico about what they'd found. On June 24, Arecibo started bistatic radar observations of 2017 YE5, where Arecibo beamed radar at the asteroid and the Green Bank Observatory (GBO) in West Virginia received the reflected signals. By June 26, the binary nature of 2017 YE5 was confirmed with two objects revolving about one another every 20 to 24 hours. Further corroboration came from the Center for Solar System Studies in Rancho Cucamonga, California.
One unexpected property of 2017 YE5 is that the two objects are as dark as charcoal, yet one object is much more radar reflective than the other – something not seen in any other of 50 binary asteroids studied by radar. NASA says this may indicate that one may have a rougher surface or a different density.
NASA sees 2017 YE5 is an opportunity to gain a better understanding of binary asteroids and their formation as well as their internal composition.
The video below discusses 2017 YE5.
Source: NASA
